Due to partial double bond character, isomerization of the peptide bond between the cis and trans states occurs at a very slow rate. This process becomes of special importance for ...X-pro...bonds in a polypeptide chain because the cis and trans forms are of nearly equal stability. Earlier studies have suggested that the rate-limiting step in the unfolding and refolding of small globular proteins is due to proline isomerization, and that the true conformational change occur on a much faster time scale. More definitive information is being sought for specific protein folding reactions. There are also suggestions that other structural processes may be controlled or influenced by proline isomerization, including subtle changes in native state structure, stereospecific synthesis of proteins on the ribosome, regulation of enzyme and hormone activity, etc.